Mechanical pulp is produced industrially by grinding or refining wood raw material. In grinding, whole tree trunks are pressed against a rotating cylindrical surface, whose surface structure is formed to detach fibres from the wood. The obtained pulp is discharged with spraywaters from the grinder to fractionation, and the reject is refined in a disc refiner. This method produces pulp that contains short fibres and scatters light well. A typical example to be mentioned of a grinding process is U.S. Pat. No. 4,381,217. In the manufacture of refiner mechanical pulp, the starting material consists of wood chips which are guided to the centre of a disc refiner, from where they are transferred by the effect of a centrifugal force and a steam flow to the circumference of the refiner while being disintegrated by the blades on the surface of the disc. Typically, multi-phase refining is necessary for obtaining finished pulp in this process. The coarse fraction separated in the process can be directed into so-called reject refining. This method produces pulp with longer fibres compared to the above-described groundwood. Refining processes have been presented in, for example, publications WO-9850623, U.S. Pat. No. 4,421,595, and U.S. Pat. No. 7,237,733.
By said methods, mechanical pulp is produced, in which the fibres of wood raw material have been separated from each other and possibly refined further, depending on the energy used. By these methods, pulp is obtained in which the fibres fall within the dimensions of wood fibres, typically having a diameter greater than 20 μm. Fibre raw material with the same particle size can be obtained by preparing chemical pulp, that is, by processing the wood raw material chemically to separate the fibres. Cellulose containing fibre raw material obtained by mechanical or chemical pulping is commonly used for manufacturing paper or cardboard products.
Wood fibres can also be disintegrated into smaller parts by removing fibrils which act as components in the fibre walls, wherein the particles obtained become significantly smaller in size. The properties of so-called nanocellulose thus obtained differ significantly from the properties of normal cellulose. By using nanocellulose, it is possible to provide a product with, for example, better tensile strength, lower porosity and at least partial translucency, compared with using cellulose. Nanocellulose also differs from cellulose in its appearance, because nanocellulose is gel-like material in which the fibrils are present in a water dispersion. Because of the properties of nanocellulose, it has become a desired raw material, and products containing it would have several uses in industry, for example as an additive in various compositions.
Nanocellulose can be isolated as such directly from the fermentation process of some bacteria (including Acetobacter xylinus). However, in view of large-scale production of nanocellulose, the most promising potential raw material is raw material of plant origin and containing cellulose fibres, particularly wood. The production of nanocellulose from wood raw material requires the decomposition of the fibres further to the size class of fibrils. In processing, a cellulose fibre suspension is run several times through a homogenizing step that generates high shear forces in the material. For example in U.S. Pat. No. 4,374,702, this is achieved by guiding the suspension under high pressure repeatedly through a narrow opening where it achieves a high speed. In U.S. Pat. No. 5,385,640; U.S. Pat. No. 6,183,596; and U.S. Pat. No. 7,381,294; in turn, refiner discs are presented, between which a fibre suspension is fed several times.
In practice, the production of nanocellulose from cellulose fibres of the conventional size class can, at present, only be implemented by disc refiners of laboratory scale, which have been developed for the needs of food industry. This technique requires several refining runs in succession, for example 5 to 10 runs, to obtain the size class of nanocellulose. The method is also poorly scalable up to industrial scale.